When to trade bishop for knight

The other reason would be to create critical pawn structure weaknesses following recaptures.

See BxN as sacrificing the minor exchange just like you see RxN as sacrificing the exchange.

Sometimes it is worth giving up a Bishop for Knight if you can ruin your opponent's pawn skeleton. This is seen in openings such as the Nimzo-Indian Defence and French: McCutcheon Variation.

I have also heard that the Queenside pieces are of less value than the Kingside, therefore occasionally swapping the Queen's Bishop for King's Knight is a worthwhile trade (the King's Knight is an excellent defender). Here is one example.

#24
I do not agree with the last position.
1 c3 or 1 Qe2 may be better than 1 Bg5.
After 1 Bg5 h6 2 Bh4 preserving the bishop's pair may be better than 2 Bxf6.
After 2 Bxf6 Qxf6 3 Qe2 c5 black opens the position for his bishop's pair.

Richter-Veresov attack is really only working if you trade your black bishop for black's knight on f6 and double their kingside pawns. 

@tygxc You are right. c3, Qe2, and Bh4 could all be better moves. However, even after 2. Bxf6 Qxf6, 3. Qe2 c5, 4. dxc5 Bxc5, White still has a +0.15 advantage. My point is that the power of the Bishop pair seems to be exaggerated, with most players not being good enough to fully capitalize with the two bishops.

#27
The bishop's pair offers a long term endgame advantage, which can win by itself. If you take from the initial position a knight away from one side and a bishop away from the other side, then the side with the bishop's pair can win.

#26
Richter - Veresov is not really worth it. The loss of the bishop's pair is worse than the doubled pawn.
https://www.chessgames.com/perl/chessgame?gid=1082377 

However, in the Richter - Rauzer Variation the trade BxN makes sense
https://www.chessgames.com/perl/chessgame?gid=1044371 

bishops tend to be better in endgames with pawns on both flanks, but when all the action is taking place in one sector of the board sometimes a knight can be the stronger piece.  🐴

As a rule of thumb if the game is closed(the pawns are locked) trade the bishops. If the game is open (the pawns have been traded) trade the knights.

#31
A closed position can sometimes open,
an open position very rarely closes.

meet you in the middle with 'usually'.

#32

#32

#32 Sorry about those 2 posts i was trying to hit Tab but i was sending them. Anyway not only is your msg very true but also very deep....lol